feat(ci): set fixed 60% coverage target for frontend in codecov

[emarkees]

PR Title

feat(ci): set fixed 60% coverage target floor for frontend in Codecov

Closes #1078

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PR Description

📝 Summary

Updates .github/codecov.yml to replace the flexible auto coverage target for the frontend status check with a fixed minimum coverage floor of 60%, aligning it with the backend target floor (60%) and patch.default target (60%).

❓ Why this matters

Previously, .github/codecov.yml set frontend.target to auto (don't regress from base commit), while backend was pinned to 60% and contracts to 70%. PRs starting from a low coverage baseline could maintain a low frontend coverage indefinitely since auto never enforced a fixed minimum coverage floor.

🛠 Changes

• Updated coverage.status.project.frontend.target in .github/codecov.yml from auto to 60%.
• Added inline documentation comment explaining the fixed 60% minimum floor.

🧪 Verification

• Validated YAML structure and formatting in .github/codecov.yml.
• Verified backend (60%), contracts (70%), and patch.default (60%) remain unchanged.